Brachytherapy for prostate cancer is mainly performed by inserting a source obtained by tightly enclosing iodine-125 of a radioactive substance in a capsule made of titanium (hereinafter, simply referred to as a source) into a prostate. A cartridge loaded with 5 or 15 of such sources is usually provided and such cartridge C is provided to be tightly enclosed in a bag under sterile conditions. Sources S are loaded in the cartridge C with their axial directions aligned (the axial directions are parallel to each other) (FIG. 15(A)).
In the brachytherapy, the number of sources inserted into a prostate and insertion positions thereof are determined depending on a state of prostate cancer of each person on the precondition that amounts of radioactivity of a radioactive substance tightly enclosed in the respective sources are the same. The number of sources to be inserted is approximately 70 to 100 for one brachytherapy.
It is, however, said that there are defective sources having an amount of radioactivity different from a nominal value suggested by a supplier of cartridges among a plurality of sources on the order of one out of several hundreds or on the order of two out of one hundred in the case of poor-quality sources. For example, a source having almost no radioactivity or a source having a larger amount of radioactivity than the nominal value may be included. When such a defective source is used, problems occur such that a desired therapeutic effect cannot be achieved due to a shortage of an exposure dose, or conversely, other tissues are affected due to an excessive exposure dose, or the like. The American Association of Physicists in Medicine (AAPM) therefore recommends that at least 10% of sources to be used or all the sources, if possible, are measured at each of the facilities.
Properly speaking, radiation intensity of all sources should be measured at respective facilities where sources are used. However, amounts of radioactivity in capsules have to be measured one by one according to generally employed radiation intensity measuring methods using an ionization chamber (radiation measuring apparatus). This brings the following disadvantages (1) to (7), and in fact, it is very difficult to measure radiation intensity of all sources at each of the facilities.    (1) A wrapped cartridge under sterile conditions needs to be taken out of a bag.    (2) Sources need to be taken out of the cartridge.    (3) The sources are measured one by one, thereby requiring a large amount of time.    (4) The sources taken out of the cartridge need to be loaded in the cartridge again.    (5) The cartridge again loaded with the sources needs to be sterilized again.    (6) It is difficult for hands and fingers of a worker to be prevented from being exposed to radiation during the operation of (1) to (5).    (7) A calibrated ionization chamber for exclusive use is required.
In order to solve the above problems, a measuring apparatus for measuring radiation intensity of each source with sources loaded in a cartridge has been developed (Patent Literature 1).
Patent Literature 1 discloses a technique related to a measuring apparatus for measuring radiation intensity of sources. The measuring apparatus includes a reception portion for receiving a cartridge loaded with encapsulated sealed radioactive sources inside thereof, and further, is provided with an insertion opening through which the cartridge is inserted into the reception portion from the outside and a plurality of apertures extending through between the reception portion and the outside.
According to the configuration, the cartridge is inserted into the reception portion of the measuring apparatus from the insertion opening, and the measuring apparatus is provided on X-ray film so that the plurality of apertures come into contact with the X-ray film. Then, radiations emitted from the respective sources leak outside the measuring apparatus via corresponding apertures. The X-ray film in contact with the measuring apparatus is then exposed to the radiations, and therefore, information about radiation intensity of each source is recorded on the film. Consequently, desired information can be obtained by analyzing the record on the film.
The measuring apparatus in Patent Literature 1 measures the sources loaded in the cartridge as they are and thereby the above problems (2) to (4) may be solved. However, the cartridge wrapped under sterile conditions cannot be measured without being taken out of a bag, leading to failure in solving the above problems (1) and (5).
Moreover, there is a high possibility that the worker is exposed to radiation because the operation is performed by taking the cartridge out of the bag, also leading to failure in solving the above problem (6).
Although the measuring apparatus in Patent Literature 1 can solve the problems (2) to (4), measuring accuracy of radiation intensity disadvantageously decreases.
In the case of the measuring apparatus in Patent Literature 1, the measuring apparatus is for exposing the X-ray film to radiations leaking from a plurality of apertures h. In order to obtain information of radiation intensity of each source S, central axes of the plurality of the sources S and central axes of the plurality of the apertures h have to be all aligned correctly so that each of the apertures h corresponds to one source (FIG. 15(B)).
However, not all the sources S loaded in the cartridge Care loaded at the same intervals and thereby there is a slight difference in placement of the sources S among respective cartridges C. Thus, when the plurality of apertures h are positioned at regular intervals according to an average diameter (0.8 mm) of the sources S, the central axes of the sources S and the central axes of the apertures h may be deviated from each other. This happens in the case where the loading intervals are different from the average diameter such as the case where sources S not having the average diameter are loaded. As a result, the radiation intensity of each of the sources S cannot be measured correctly (FIG. 15 (C)).
As described above, the measuring apparatus in Patent Literature 1 cannot solve the conventional problems (1) to (7), and therefore, a development of a measuring apparatus capable of solving the problems has been desired.
[Citation List]
[Patent Literature]
Patent Literature 1: Japanese Utility Model Registration No. 3132529